1. Field of the Invention
The invention relates to curtain coater apparatus and more particularly, to apparatus for coating objects with fluid material by passing the objects through a curtain of the material.
2. Description of the Prior Art
A problem associated with conventional curtain coater application heads, whether their curtain is formed by forcing coating material through an orifice, or over a weir, is that they contain pools that permit solids to precipitate from many coating materials used in coating objects. The relatively large volume of coating material that is required to fill a conventional curtain coater reservoir, pumping unit, and coating head, must be re-circulated many times as it is slowly used. Multiple re-circulation of a large volume of coating material causes degradation of the material. The two exposed surfaces of the curtain, and force circulation of air that safety precautions require for solvent vapor removal, necessitates on-line mixing to maintain proper solid content for process control. However, accurate monitoring of viscosity is complicated by evaporation cooling. Furthermore, it becomes unreliable as an indicator of solid content if viscosity changes as a result of the mechanical agitation and heat generated by re-circulation of coating material through a pump, filter, and a curtain-forming orifice. Significant, too, are evaporation rates of the constituents of a blended solvent. As evaporated solvent is replaced in response to an increase in viscosity, the system becomes progressively richer in that constituent which evaporates more slowly. Because a given volume of one constituent has a different effect on viscosity than another, viscosity again becomes a poor indicator of solid content. Moreover, as the system becomes richer in the slower evaporating solvent constituent, its drying characteristics change. Conventional curtain coaters generally are indifferent to such time-associated deterioration of coating materials.
Prior curtain coaters also failed to provide for safety of operation. The conveyors of such apparatus characteristically travel at relatively high speed and have considerable inertia, but are not provided with means for emergency stops. In this regard, the positive pumping units utilized in such prior curtain coaters were not provided with relief valves. Relief valves were omitted because of the clean-up problem involved in their use, and where hot-melt coating materials were used, the valves ceased to function when most needed, for example, when the coating materials became cold and solidified.
Another major problem associated with prior art curtain coaters is that of turbulence created at the point of entry of the coating material into the curtain coater heads, which adversely affects the uniformity of the curtain produced.
A large percentage of the operator's time is spent in cleaning a conventional coater. Rather than simplifying the clean-up process, conventional coaters have been provided with multiple coater heads, pivoting over single, or multiple troughs, or with exchangeable center sections, so that the time consuming clean-up process can take place off-line.
Moreover, when one considers that a conventional coater is cleaned by filling it with solvent and re-circulating, then draining, refilling, and re-circulating until it is washed clean, the magnitude of the fire hazard, the air pollution, and the health hazard to the operator can be readily appreciated.
Example of prior art devices are revealed in U.S. Pat. Nos. 2,935,424; 2,963,002; 2,976,837; 3,067,060; 3,088,633; 3,132,968; 3,205,089; 3,299,195 and 3,468,099.